The present invention relates to a method and device for regulating a braking operation for an individual wheel on a vehicle equipped with a braking control system. The system comprises measuring means for monitoring wheel speed, an electronic unit, and a valve unit situated in the brake line for coupling the brake pressure medium to an individual wheel. In the method according to the invention, the braking pressure to the wheel is caused to vary in response to reference values for different control parameters, so that during the course of braking the wheel is affected during a number of braking control cycles, during which the wheel, by means of pressure alterations in the braking pressure medium, is retarded from a first rotational speed to a second lower rotational speed and is thereafter allowed to increase its rotational speed at a reduced braking pressure, so that at least one reference parameter sets off the next controlling cycle, the maximum braking capacity of the wheel on the appropriate road surface being reached and exceeded during the reduction of the wheel rotational speed from the first to the second value.
In braking, it is a well known problem that when the braking moment on a wheel becomes larger than the road moment which is the product of wheel radius and frictional force between wheel and road, the wheel will skid. The result of this may be that the braking capacity of the wheel is decreased simultaneously as steering capacity is wholly or partly lost.
The braking force which is obtained in the contact between wheel and substructure during braking is dependent on many factors, such as the nature of the substructure and the wheel, the speed of the vehicle, wheel loading, prevailing temperature etc. For each combination of such conditions the frictional conditions can be described by a characteristic graph in a coordinate system which defines braking force on the wheel as a function of the wheel slip. "Slip" in this case means the difference between the speed of the vehicle and the wheel in relation to the speed of the vehicle. By "wheel speed" is meant here, and in what follows, the rotational speed of the wheel. Characteristic for such a braking force graph is that braking force is present only when slip deviates from zero. With increasing slip, the braking force increases to a maximum, whereafter it diminishes once again. Graphs may also be plotted in the same coordinate system to show how the ability of a wheel to take up side forces varies with the slip. The ability to take up side forces is greatest when no slip prevails, and diminishes first slowly and then more and more rapidly with increasing slip. It is a generally accepted fact that the ability to take up side forces when the lag corresponding to the maximum on the braking force graph still has an acceptable value. To obtain optimal braking capacity it is therefore important to control the braking function at and in the vicinity of the maximum on the braking force graph. With braking pressure control it is also important to keep the braking pressure variations required for the braking function within acceptable limits in the respective braking circuits. This applies especially to pneumatic braking systems, since air consumption would otherwise be too great.